1. Field of the Invention
The present invention relates to a miniaturized multi-layer coplanar wave guide low pass filter, and more particularly, to a miniaturized multi-layer coplanar wave guide low pass filter which is capable of enlarging the region of the characteristic impedance of a transmission line and miniaturizing the size of a filter by utilizing a multi-layer coplanar wave guide.
2. Description of the Prior Art
As it is well known, a filter is essentially composed of series inductors and parallel capacitors.
A low pass filter plays an important role on the microwave circuit, and is used to eliminate noises in a frequency change over circuit. However, the operation frequency in a new generation mobile communication system has been raised up to 30 GHz and above so as to cope with the trend of rapid development of the modern radio communication technology, and the design of transmission and distribution mode should be considered to operate the electromagnetic wave having the frequency 30 GHz and above.
For achieving the aforesaid object, the design of a well-known coplanar wave-guide structure which has been presented by Mr. C. P. Wen in 1969 is used. Its basic construction is shown in FIG. 1 in which the conductor and the grounding plate are made of a metal including gold, copper, etc. Meanwhile, the normal size of T.H.G.H is remained uncertain and is left it to the manufacture's decision, thereby having the merits of obtaining a simple coplanar structure with other active components, and omitting the processes of backside metallization and via hole forming so as to facilitate the fabrication process. Besides, the characteristic impedance is determined by the proportion of width between vacant slot and signal wire so that its electrical properties are less influenced by the thickness of the substrate. Moreover, as it is a real simple coplanar structure whose transmission line, signal wire, and ground wire is in the same plane so that cascading and paralleling among circuits can be easily performed by omitting formation of extra via holes which is necessary in a microstrip transmission line. The above mentioned via holes tend to bring about innegligible inductance effect at high frequency resulting in reducing efficiency of the circuit. Eliminating vertical via holes not only leads to reduction of circuit fabrication cost, but also enables to improve the selectivity of the circuit layout.
In bygone time, the two dimensional mode is usually employed when designing a coplanar wave guide low pass filter as shown in FIG. 2, wherein the equalizing potential among metal parts is mainly performed by means of bonding or air bridging. However, it is found to be impracticable as the area occupied by these circuits is considerably large. Therefore, it is an urgent matter as to effectively minimizing the area occupied by a circuit when designing a low pass filter. Accordingly, the inventor of the present invention disclosed a two-dimensional coplanar wave guide low pass filter based on the principle of step type impedance as shown in FIG. 3. It can be seen that such a two-dimensional coplanar wave guide low pass filter has a merit capable of saving process of bonding and forming air bridges among metal parts. Furthermore, for an equivalent circuit, a five order low pass filter was disclosed according to Butterworth model as shown in FIG. 4.
In the meanwhile, it is found the area of this two-dimensional coplanar wave guide low pass filter is still unable to be reduced to a satisfactory extent which complies with the compactness of light, thin, short, and small that fulfils the current radio communication system's needs.
In addition, the conventional coplanar wave guide filter has a characteristic impedance in the range of 50˜70Ω which is considered one of the disadvantages when it is to be in match with the network.
Aiming at the above-depicted defects, the present invention is to propose an innovative miniaturized multi-layer coplanar low pass filter after long time efforts made by the present inventor which is capable of eliminating the disadvantages inherent to the conventional products.